A global positioning system (GPS) is widely known as a system for positioning, for example, in car navigation systems. The GPS is a positioning system so designed that the apparent distances between GPS satellites and a GPS receiving device are measured and the position of the receiving device is computed. (For example, refer to Patent Document 1.) This receiving device receives 1575.42 MHz signals designated as L1. Signals of 1227.6 MHz and 1176.45 MHz will be available in the future, respectively, as a second signal (L2C) for civilian applications and a third signal (L5C) for civilian applications. In addition, the Galileo system under development in Europe will be available. In the Galileo system as well, signals of a plurality of frequencies are provided. When a plurality of these signals can be utilized, the performance of receiving devices can be enhanced further than ever. (For example, the reception area can be widened, and the positioning accuracy can be enhanced.)                Patent Document 1: JP-H06-0347387 A        
However, the present GPS receiving devices involve a problem. As described above, the present GPS receiving device is designed for one kind of reception signal, and thus so constituted as to accommodate only the frequency and bandwidth of that signal. FIG. 9 illustrates an example of the constitution of the GPS receiving device 501. The GPS receiving device 511 illustrated in FIG. 9 includes: an antenna 511 for receiving GPS signals; an RF amplifier unit 521 which amplifies the GPS signals received by the antenna 511; a mixer unit 531 which carries out frequency conversion; a local oscillator 541 used in frequency conversion; a reference quartz oscillator 551; an IF amplifier unit 561 which amplifies intermediate frequencies; an IF filter unit 571 for intermediate frequencies; and a digital processor unit 581 which performs signal processing. The GPS receiving device 501 operates as follows: GPS signals transmitted from GPS satellites are received by the antenna unit 511, and these GPS signals are amplified by the RF amplifier unit 521. The frequencies of the signals are converted into intermediate frequencies by the mixer unit 531. The signals are amplified by the IF amplifier unit 561, and processed by the digital processor unit 581.
Because of the above-mentioned constitution, the GPS receiving device 501 illustrated in FIG. 9 cannot receive signals of different frequency or bandwidth, such as “GPS-L2C,” “GPS-L5C,” and “Galileo” scheduled to become available in the future. To accommodate these signals that will newly become available in the future, a processing system that supports the new signal frequencies and bandwidths will be required. To attain this object, there is a problem. More specific description will be given. For example, the processing system including the mixer unit, local oscillator, reference quartz oscillator, IF amplifier unit, and IF filter unit, illustrated in FIG. 9 will be required by a number equivalent to the number of signals to be received. To support all the signals, the scale of the receiving device must be increased.
To detect a position utilizing GPS, two methods are used: two-dimensional positioning in which signals are received from three GPS satellites, and three-dimensional positioning in which signals are received from four or more GPS satellites. Positions cannot be detected only by two or less satellite. For this reason, a problem arises. In the same place, positioning can be carried out in some periods of time and cannot in other periods of time depending on the relation between the positions of GPS satellites and the present position and the environment around the present position (e.g. valley between tall buildings). In some periods of time, signals can be received from three or more satellites, and in other periods of time, signals can be received only from two or less satellite.
Another problem exists. In high-latitude land areas, the number of satellites from which signals can be received is small, and the present position is often difficult to determine.